Look — live part of brain (4)
I believe the answer is:
lobe
'part of brain' is the definition.
(I've seen this before)
'look live' is the wordplay.
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'live' becomes 'be' (synonyms).
'lo'+'be'='LOBE'
(Other definitions for lobe that I've seen before include "Part of ear or leaf" , "part of brain" , "Lower part of the ear" , "Roundish projecting part" , "could be site of a 24 [STUD]" .)